The present invention relates to a hydraulic control device for an automatic transmission in an automobile.
Hydraulic control devices that can prevent creeping movement of a vehicle equipped with a hydrodynamic-mechanical automobile transmission when the engine is idling and a forward gear of the transmission is engaged are known. See, for example, U.S. Pat. No. 4,331,045. The device disclosed in U.S. Pat. No. 4,331,045 has a shift valve which can selectively vent the pressurized fluid line leading to the forward clutch element, so as to disengage the element. The shift valve is actuated by a control valve, which is preferably formed as part of the accelerator kick-down valve, normally present in transmission control systems, to cause the transmission element to disengage when the accelerator pedal is released. The device has a line carrying an accelerator-pedal-position-dependent pressure connected to the shift valve; this line is connected by a throttle point with a pressure line carrying an engine-load-dependent pressure; this line is also connected by the control valve with a pressure medium outlet when the accelerator pedal is released.
The control valve, i.e. the kick-down valve, of the device of U.S. Pat. No. 4,331,045 has a control piston which is disposed within a bore of the piston of the kick-down valve. The bore is connected with the line leading to the shift valve. The control piston can be displaced within normal play associated with the idling position of the accelerator. When the accelerator is released, the control piston connects the bore with the outlet to vent the line leading to the shift valve. When the accelerator is depressed slightly, the control valve is pushed into the bore so as to block the outlet.
When the accelerator is depressed, a slight delay occurs before the shift valve shifts to re-engage the transmission. This delay occurs due to the time necessary to fill the hydraulic lines leading to the shift valve with pressurized fluid. As noted above, the pressurized fluid is delivered by way of a throttle point. Under normal restarting, when the accelerator is depressed moderately, i.e. below a definite depressing velocity, this known hydraulic control device operates satisfactorily. Any delay in re-engagement of the transmission element occurs while the throttle is still in its idling position, or at most is very slight, and thereby does not delay the initiation of forward movement of the vehicle in a remarkable way. However, if the accelerator pedal is depressed rapidly and comparatively strongly after a period during which it was released, a greater delay will occur before the gearshift element re-engages. This phenomenon is due to the fact that when the kick-down piston of the control valve is actuated, there is an increase in the space that must be filled with a pressure medium from the pressure line, and the space is filled relatively slowly through the throttle point in the pressure line. Consequently, the pressure required to move the shift valve to a position to re-engage the transmission builds up only with some delay in the line leading to the shift valve, and this can delay the start of forward vehicle movement.